This invention relates generally to fabrics useful as covering materials for various types of frame structures and for other purposes, and more particularly to such fabrics of high strength, lightweight character especially suitable for use in the covering of aircraft frames.
For many decades, fabrics of one sort or another have been employed as airframe covering materials. Since about the mid-fifties, polyester fabrics have been widely utilized for this purpose. A suitable fabric for such usage must meet certain strength and elongation standards set by the Federal Aviation Administration (FAA), and in order to do this, the polyester fabrics heretofore employed have been woven from threads formed from relatively thick filaments which has resulted in coarse, fairly heavy fabric materials. Polyester fiber was initially defined by the Federal Trade Commission (which classifies and controls the marketing of fibers in the United States) as "a manufactured fiber in which the fiber-forming substance is any long-chain polymer composed of at least 85% by weight of an ester of dihydric alcohol and terephthalic acid." In the late 70's, however, the FTC amended the definition to read: "polyester is a manufactured fiber in which the fiber-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of an ester of the substituted aromatic carboxylic acids, including but not restricted to substituted terephthalate units and parasubstituted hydroxybenzoate units."
In the production of polyester fabric, hot, molten material of suitable polymeric composition is first extruded through a spinneret to form it into filaments. The filaments, after cooling, are heated and stretched to reduce their size to a desired denier and strengthen them through alignment of their molecules along the filament axes (the denier of a filament being the weight in grams of a segment thereof 9,000 meters long). The stretched filaments are combined into threads, cooled under tension and then spooled and routed to weaving mills where they are woven into fabric after being coated with a lubricant to prevent wear and breakage during the weaving procedure. The woven fabric, as it comes from the looms, is referred to a greige goods.
Heretofore, polyester fabrics have been manufactured in large quantities for use in the apparel industry where they are formed into various articles of clothing. Where a polyester greige fabric is to be so employed, it is first heated in air at from about 350.degree. to about 375.degree. F. under controlled tension to stabilize its filaments, then further processed in various ways for numerous applications. The polyesters heretofore employed for airframe covering purposes are the same chemically as polyester apparel fabrics, but they are made physically strong enough to meet the above-mentioned FAA requirements. This, as previously indicated, results in a material of relatively heavy and coarse-textured character, which characteristics detract from its overall effectiveness as an airframe covering material.
Various methods of covering an aircraft frame with a heat-shrinkable fabric such as polyester greige fabric are well known to those skilled in the art and need not be described in detail here. All such methods involve the basic steps of fastening the fabric to the frame and then heating it to shrink the fabric filaments and thereby cause the fabric cover to be pulled taut on the frame. The preferred heating temperature is about 350.degree. F., this having been found optimum for the development of suitable tension in the polyester filaments. Other temperatures, within certain limits, will also result in shrinkage of the polyester filaments. Temperatures of 375.degree. F. and above, however, have been found to soften the filaments and cause them to release their tension.
Those polyester fabrics heretofore employed as airframe covering materials have all been formed from filaments classified as "regular tenacity" filaments, typical examples of which normally exhibit an elongation up to 40% before breaking. This amount of stretch is undesirable in an airframe cover where a strength of 80 pounds per inch and an elongation of only 14% at 70 pounds per inch load is required under the FAA standards mentioned above. It is therefore necessary that the polyester thread filaments be of large enough size, and the thread count high enough, to meet these high strength, low elongation requirements. Thus, the smallest filament for aircraft cover suitability has heretofore been found to be of 150-denier size. Tests have shown that a fabric woven with threads formed from 34, 150-denier filaments and having 66 threads per inch, warp and fill, will narrowly pass the minimum FAA requirements for airframe cover utility. Such a fabric has a weight of approximately 2.7 oz. per square yard.